The availability of viable nanofabrication processes is a key factor to realizing the potential of nanotechnologies. In particular, the availability of viable nanofabrication processes is important to the fields of photonics, electronics, and proteomics. Traditional imprint lithographic (IL) techniques are an alternative to photolithography for manufacturing integrated circuits, micro- and nano-fluidic devices, and other devices with micrometer and/or nanometer sized features. There is a need in the art, however, for new materials to advance IL techniques. See Xia, Y., et al., Angew. Chem. Int. Ed., 1998, 37, 550-575; Xia, Y., et al., Chem. Rev., 1999, 99, 1823-1848; Resnick, D. J., et al., Semiconductor International, 2002, June, 71-78; Choi, K. M., et al., J. Am. Chem. Soc., 2003, 125, 4060-4061; McClelland, G. M., et al., Appl. Phys. Lett., 2002, 81, 1483; Chou, S. Y., et al., J. Vac. Sci. Technol. B, 1996, 14, 4129; Otto, M., et al., Microelectron. Eng., 2001, 57, 361; and Bailey, T., et al., J. Vac. Sci. Technol., B, 2000, 18, 3571.
Imprint lithography comprises at least two areas: (1) soft lithographic techniques, see Xia, Y., et al., Angew. Chem. Int. Ed., 1998, 37, 550-575, such as solvent-assisted micro-molding (SAMIM); micro-molding in capillaries (MIMIC); and microcontact printing (MCP); and (2) rigid imprint lithographic techniques, such as nano-contact molding (NCM), see McClelland, G. M., et al., Appl. Phys. Lett., 2002, 81, 1483; Otto, M., et al., Microelectron. Eng., 2001, 57, 361; “step and flash” imprint lithographic (S-FIL), see Bailey, T., et al., J. Vac. Sci. Technol., B, 2000, 18, 3571; and nanoimprint lithography (NIL), see Chou, S. Y., et al., J. Vac. Sci. Technol. B, 1996, 14, 4129.
Polydimethylsiloxane (PDMS) based networks have been the material of choice for much of the work in soft lithography. See Quake, S. R., et al., Science, 2000, 290, 1536; Y. N. Xia and G. M. Whitesides, Angew. Chem. Int. Ed. Engl. 1998, 37, 551; and Y. N. Xia, et al., Chem. Rev. 1999, 99, 1823.
The use of soft, elastomeric materials, such as PDMS, offers several advantages for lithographic techniques. For example, PDMS is highly transparent to ultraviolet (UV) radiation and has a very low Young's modulus (approximately 750 kPa), which gives it the flexibility required for conformal contact, even over surface irregularities, without the potential for cracking. In contrast, cracking can occur with molds made from brittle, high-modulus materials, such as etched silicon and glass. See Bietsch, A., et al., J. Appl. Phys., 2000, 88, 4310-4318. Further, flexibility in a mold facilitates the easy release of the mold from masters and replicates without cracking and allows the mold to endure multiple imprinting steps without damaging fragile features. Additionally, many soft, elastomeric materials are gas permeable, a property that can be used to advantage in soft lithography applications.
Although PDMS offers some advantages in soft lithography applications, several properties inherent to PDMS severely limit its capabilities in soft lithography. First, PDMS-based elastomers swell when exposed to most organic soluble compounds. See Lee, J. N., et al., Anal. Chem., 2003, 75, 6544-6554. Although this property is beneficial in microcontact printing (MCP) applications because it allows the mold to adsorb organic inks, see Xia, Y., et al., Angew. Chem. Int. Ed., 1998, 37, 550-575, swelling resistance is critically important in the majority of other soft lithographic techniques, especially for SAMIM and MIMIC, and for IL techniques in which a mold is brought into contact with a small amount of curable organic monomer or resin. Otherwise, the fidelity of the features on the mold is lost and an unsolvable adhesion problem ensues due to infiltration of the curable liquid into the mold. Such problems commonly occur with PDMS-based molds because most organic liquids swell PDMS. Organic materials, however, are the materials most desirable to mold. Additionally, acidic or basic aqueous solutions react with PDMS, causing breakage of the polymer chain.
Secondly, the surface energy of PDMS (approximately 25 mN/m) is not low enough for soft lithography procedures that require high fidelity. For this reason, the patterned surface of PDMS-based molds is often fluorinated using a plasma treatment followed by vapor deposition of a fluoroalkyl trichlorosilane. See Xia, Y., et al., Angew. Chem. Int. Ed., 1998, 37, 550-575. These fluorine-treated silicones swell, however, when exposed to organic solvents.
Third, the most commonly-used commercially available form of the material used in PDMS molds, e.g., Sylgard 184® (Dow Corning Corporation, Midland, Mich., United States of America) has a modulus that is too low (approximately 1.5 MPa) for many applications. The low modulus of these commonly used PDMS materials results in sagging and bending of features and, as such, is not well suited for processes that require precise pattern placement and alignment. Although researchers have attempted to address this last problem, see Odom, T. W., et al., J. Am. Chem. Soc., 2002, 124, 12112-12113; Odom, T. W. et al., Langmuir, 2002, 18, 5314-5320; Schmid, H., et al., Macromolecules, 2000, 33, 3042-3049; Csucs, G., et al., Langmuir, 2003, 19, 6104-6109; Trimbach, D., et al., Langmuir, 2003, 19, 10957-10961, the materials chosen still exhibit poor solvent resistance and require fluorination steps to allow for the release of the mold.
Rigid materials, such as quartz glass and silicon, also have been used in imprint lithography. See Xia, Y., et al., Angew. Chem. Int. Ed., 1998, 37, 550-575; Resnick, D. J., et al., Semiconductor International, 2002, June, 71-78; McClelland, G. M., et al., Appl. Phys. Lett., 2002, 81, 1483; Chou, S. Y., et al., J. Vac. Sci. Technol. B, 1996, 14, 4129; Otto, M., et al., Microelectron. Eng., 2001, 57, 361; and Bailey, T., et al., J. Vac. Sci. Technol., B, 2000, 18, 3571; Chou, S. Y. et al., Science, 1996, 272, 85-87; Von Werne, T. A., et al., J. Am. Chem. Soc., 2003, 125, 3831-3838; Resnick, D. J., et al., J. Vac. Sci. Technol. B, 2003, 21, 2624-2631. These materials are superior to PDMS in modulus and swelling resistance, but lack flexibility. Such lack of flexibility inhibits conformal contact with the substrate and causes defects in the mask and/or replicate during separation.
Another drawback of rigid materials is the necessity to use a costly and difficult to fabricate hard mold, which is typically made by using conventional photolithography or electron beam (e-beam) lithography. See Chou, S. Y., et al., J. Vac. Sci. Technol. B, 1996, 14, 4129. More recently, the need to repeatedly use expensive quartz glass or silicon molds in NCM processes has been eliminated by using an acrylate-based mold generated from casting a photopolymerizable monomer mixture against a silicon master. See McClelland, G. M., et al., Appl. Phys. Lett., 2002, 81, 1483, and Jung, G. Y., et al., Nanoletters, 2004, ASAP. This approach also can be limited by swelling of the mold in organic solvents.
Despite such advances, other disadvantages of fabricating molds from rigid materials include the necessity to use fluorination steps to lower the surface energy of the mold, see Resnick, D. J., et al., Semiconductor International, 2002, June, 71-78, and the inherent problem of releasing a rigid mold from a rigid substrate without breaking or damaging the mold or the substrate. See Resnick, D. J. et al., Semiconductor International, 2002, June, 71-78; Bietsch, A., J. Appl. Phys., 2000, 88, 4310-4318. Khang, D. Y., et al., Langmuir, 2004, 20, 2445-2448, have reported the use of rigid molds composed of thermoformed Teflon AF® (DuPont, Wilmington, Del., United States of America) to address the surface energy problem. Fabrication of these molds, however, requires high temperatures and pressures in a melt press, a process that could be damaging to the delicate features on a silicon wafer master. Additionally, these molds still exhibit the intrinsic drawbacks of other rigid materials as outlined hereinabove.
Further, a clear and important limitation of fabricating structures on semiconductor devices using molds or templates made from hard materials is the usual formation of a residual or “scum” layer that forms when a rigid template is brought into contact with a substrate. Even with elevated applied forces, it is very difficult to completely displace liquids during this process due to the wetting behavior of the liquid being molded, which results in the formation of a scum layer. Thus, there is a need in the art for a method of fabricating a pattern or a structure on a substrate, such as a semiconductor device, which does not result in the formation of a scum layer.
The fabrication of solvent resistant, microfluidic devices with features on the order of hundreds of microns from photocurable perfluoropolyether (PFPE) has been reported. See Rolland, J. P., et al., J. Am. Chem. Soc., 2004, 126, 2322-2323. PFPE-based materials are liquids at room temperature and can be photochemically cross-linked to yield tough, durable elastomers. Further, PFPE-based materials are highly fluorinated and resist swelling by organic solvents, such as methylene chloride, tetrahydrofuran, toluene, hexanes, and acetonitrile among others, which are desirable for use in microchemistry platforms based on elastomeric microfluidic devices. There is a need in the art, however, to apply PFPE-based materials to the fabrication of nanoscale devices for related reasons.
Further, there is a need in the art for improved methods for forming a pattern on a substrate, such as method employing a patterned mask. See U.S. Pat. No. 4,735,890 to Nakane et al.; U.S. Pat. No. 5,147,763 to Kamitakahara et al.; U.S. Pat. No. 5,259,926 to Kuwabara et al.; and International PCT Publication No. WO 99/54786 to Jackson et al., each of which is incorporated herein by reference in their entirety.
There also is a need in the art for an improved method for forming isolated structures that can be considered “engineered” structures, including but not limited to particles, shapes, and parts. Using traditional IL methods, the scum layer that almost always forms between structures acts to connect or link structures together, thereby making it difficult, if not impossible to fabricate and/or harvest isolated structures.
There also is a need in the art for an improved method for forming micro- and nanoscale charged particles, in particular polymer electrets. The term “polymer electrets” refers to dielectrics with stored charge, either on the surface or in the bulk, and dielectrics with oriented dipoles, frozen-in, ferrielectric, or ferroelectric. On the macro scale, such materials are used, for example, for electronic packaging and charge electret devices, such as microphones and the like. See Kressman, R., et al., Space-Charge Electrets, Vol. 2, Laplacian Press, 1999; and Harrison, J. S., et al., Piezoelectic Polymers, NASA/CR-2001-211422, ICASE Report No. 2001-43. Poly(vinylidene fluoride) (PVDF) is one example of a polymer electret material. In addition to PVDF, charge electret materials, such as polypropylene (PP), Teflon-fluorinated ethylene propylene (FEP), and polytetrafluoroethylene (PTFE), also are considered polymer electrets.
Further, there is a need in the art for improved methods for delivering therapeutic agents, such as drugs, non-viral gene vectors, DNA, RNA, RNAi, and viral particles, to a target. See Biomedical Polymers, Shalaby, S. W., ed., Harner/Gardner Publications, Inc., Cincinnati, Ohio, 1994; Polymeric Biomaterials, Dumitrin, S., ed., Marcel Dekkar, Inc., New York, N.Y., 1994; Park, K., et al., Biodegradable Hydrogels for Drug Delivery, Technomic Publishing Company, Inc., Lancaster, Pa., 1993; Gumargalieva, et al., Biodegradation and Biodeterioration of Polymers: Kinetic Aspects, Nova Science Publishers, Inc., Commack, N.Y., 1998; Controlled Drug Delivery, American Chemical Society Symposium Series 752, Park, K., and Mrsny, R. J., eds., Washington, D.C., 2000; Cellular Drug Delivery: Principles and Practices, Lu, D. R., and Oie, S., eds., Humana Press, Totowa, N.J., 2004; and Bioreversible Carriers in Drug Design: Theory and Applications, Roche, E. B., ed., Pergamon Press, New York, N.Y., 1987. For a description of representative therapeutic agents for use in such delivery methods, see U.S. Pat. No. 6,159,443 to Hallahan, which is incorporated herein by reference in its entirety.
In sum, there exists a need in the art to identify new materials for use in imprint lithographic techniques. More particularly, there is a need in the art for methods for the fabrication of structures at the tens of micron level down to sub-100 nm feature sizes.